Vertical Bar in Redhead Eye: What is this?

Nature Talk
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Hi All,

I have a question about a photo I took of a Redhead (duck).

Looking at the 4th photo in this series: Redhead (Aythya americana) from Tanque Verde, AZ, USA on February 21, 2026 at 05:23 PM by Ron Bondy. Beautiful redhead in the golden hour light at Agua Caliente Regional Park in Tucson, AZ, USA · iNaturalist

There is a vertical line in the eye… can anyone explain to me what this is? I only see it occasionally in my photographs.

Thanks,

Ron

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nictitating membrane, probably. lots of birds have this: https://www.inaturalist.org/observations/262879011

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I did some more checking, it looks like it might be a vertical specular highlight.. i.e. just a reflection. I see this vertical bar infrequently so I’m curious about it. I shot these redhead pics at 6 frames per second and the frame before and after do not have the vertical bar.

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https://www.inaturalist.org/observations?project_id=nictitating-membrane-deployed&taxon_id=7056

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As @pisum says, it is the nictitating membrane. Mose specifically, the nictitating membrane is casting a narrow shadow onto the iris and pupil. It is a very cool photo!

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Thanks!

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